1. Field of the Invention
The present invention relates to a positive temperature coefficient thermistor heater and a positive temperature coefficient thermistor heater device which are useful as a heater, etc. for generating hot air in a heater, a dryer and so on.
2. Description of the Related Art
Recently, a positive temperature coefficient (hereinafter, often abbreviated as "PTC") thermistor heater has become widely used as a heating element of a hot air type heater, a bedclothes dryer, a hair dryer or the like. A PTC thermistor heater has a positive temperature characteristic. When such a thermistor heater is self-heated to a temperature higher than a certain level, a resistance therein is increased and the thermistor heater exhibits a self-regulating property so as to be kept at the temperature. For example, a hot air type heater utilizes this property. Specifically, such a heater has a PTC thermistor heater device in which a radiator plate is in thermal contact with plural PTC thermistor heaters which are electrically connected to each other. When the temperature of hot air generated by the hot air type heater is to be changed, the output of the PTC thermistor heater device is changed by increasing or decreasing the number of PTC thermistor heaters to be energized.
A conventional PTC thermistor heater will be described with reference to FIG. 1. A PTC thermistor heater device 60 comprises PTC thermistor heaters 61, radiator plates 62, and a first terminal 63 and second terminals 64 which are conductor terminals.
Each of the PTC thermistor heaters 61 is configured by forming electrodes (not shown) on both principal faces of a substantially rectangular plate-like PTC thermistor element. Each of the radiators 62 is a corrugated thin plate made of aluminum which is excellent in heat conduction and having a width equal to that of the PTC thermistor heaters 61.
In one unit 60a of the PTC thermistor heater device 60 consisting of these components, a plural number (in the illustrated example, 4) of the PTC thermistor heaters 61 are horizontally arranged, and electrically connected to each other and in thermal contact with each other to accomplish excellent heat conduction, by disposing two radiators 62 respectively on the both principal faces of the combination of the thermistor heaters so as to be connected with the electrodes of the PTC thermistor heaters 61. A first terminal 63 is disposed at one end of one of the radiators 62, and a second terminal 64 at one end of the other radiator 62. The radiators 62 are designed so as to have opposite polarities through the PTC thermistor heaters 61. The PTC thermistor heater device 60 is configured by connecting in parallel two sets of the units 60a each of which is one of the units constituting the PTC thermistor heater device to each other so that their first terminals 63 are brought in contact with each other back to back.
In the thus configured PTC thermistor heater device 60, when the first terminals 63 and the second terminals 64 are connected to a power source, the electrodes on the both principal faces of PTC thermistor heaters 61 are energized through the radiators 62 so that the PTC thermistor heaters 61 generate heat. The heat generated by the PTC thermistor heaters 61 is dissipated to the outside through the radiators 62, with the result that the PTC thermistor heater device 60 functions as a heater. When the first terminals 63 are used as a common terminal and the second terminals 64 are connected to a power source, the PTC thermistor heater device 60 generates heat of the maximum output level. When the first terminals 63 and one of the second terminals 64 are connected to the power source, the heat output can be reduced to a 1/2 of the maximum output level.
Japanese Patent Unexamined Publication No. Hei 4-251901 discloses a PTC thermistor heater device in which a single heating element formed by connecting a plurality of PTC thermistor heaters can change its output by itself. The PTC thermistor heater device is configured so that at least one of electrodes of the plurality of PTC thermistor heaters is provided with a plurality of power supply electrodes for selectively joining the plurality of PTC thermistor heaters, and electrical junctions of the power supply electrodes for the same PTC thermistor heater are prevented from overlapping with each other. The electrodes of the PTC thermistor heaters, and the power supply electrodes are suitably joined with each other by a conductive adhesive and an insulative adhesive. In the thus configured PTC thermistor heater device, the output can be changed by selecting power supply electrodes which are to be energized so as to increase or decrease the number of PTC thermistor heaters to be energized.
However, the conventional PTC thermistor heater device shown in FIG. 1 in which the output level can be switched has a problem in that the number of selectable levels of heat generation is restricted to two, that is, 100% or 50%.
In the device of Japanese Patent Unexamined Publication No. Hei 4-251901, the PTC thermistor heaters are connected to the electrode terminals by using a conductive adhesive and an insulative adhesive. Since the coefficient of thermal expansion of the PTC thermistor heaters is different from that of the power supply electrodes which are made of a metal, the thermal expansion difference between components due to heat cycle cannot be accommodated by the adhesives, thereby producing a problem in that the joining portions are peeled off. Since the temperature of the PTC thermistor heaters is elevated to a level as high as 200.degree. C., moreover, there arises a further problem in that the adhesives are deteriorated because of the high temperature.